The hydrodynamics associated with the flow of air, water, or any fluid at the junction of two surfaces is common to many engineering and natural structures. At the junction of two solid surfaces, a highly unsteady and turbulent flowfield is established when a fluid flowing over a surface encounters an obstacle. See R. L. Panton, Incompressible Flow, (John Wiley & Sons 1984), which is hereby incorporated by reference. This fluid dynamic phenomenon (e.g., the unsteady and turbulent flowfield), has been referred to in the field as the horseshoe vortex or the necklace vortex, and for consistency of reference, the term "necklace vortex" will be used herein which is intended to broadly encompass the phenomenon.
Occurrences of the necklace vortex are found in many manmade and naturally occurring circumstances and can have adverse and hazardous effects from its presence. For example, the necklace vortex can be found at the river floor junction with a bridge pier, the junction of a submarine hull and its sailplanes, the windward side of an architectural structure such as a building, the junction of inlet struts in hydrodynamic inlets such as in airbreathing engines and intake systems of industrial power plants, the junction of rotor and stator blades in turbines and compressors, the junctions of an aircraft fuselage body and wing, and in large scale geological flows at the foot of hills, mountains, and volcanoes. In each case the necklace vortex may cause adverse effects, some of which are discussed below. See also Bushnell, AERONAUTICAL JOURNAL, "Longitudinal Vortex Control--Techniques and Applications" (October 1992), at pp. 293-312.
Bridge Pier Erosion
The floor of rivers are typically composed of soil that can be transported by various shearing and mixing forces associated with the local flow of the river. The necklace vortex that forms at bridge supports and piers causes highly unsteady fluid mixing at the windward side and adjacent side regions of these obstacles. This unsteady flow causes erosion not only of the piers and supports but also of the adjacent riverbed, and sediment from the soil is forced into the river flow near the floor and then carried downstream. This event is known as scouring and can cause severe degradation of bridge foundations, ultimately leading to bridge failure and collapse. This degradation of bridge structural support has long been recognized as a leading issue in public safety, and it is estimated that damages from scouring are responsible for up to 100 million dollars in the United States each year. Thousands of bridges in the United States and abroad are known to be "scour critical." See Rhodes and Trent, "Economics of Floods, Scour, and Bridge Failures," Proceedings of the ASCE 1993 National Conference on Hydraulics Engineering, Vol. I, San Francisco, Calif. (Jul. 25-30 1993), at pp. 928-933; Young, "Risk Cost for Scour at Unknown Foundations," Proceedings of the ASCE Water Forum '92, Baltimore, Md. (August 1992), and "National Bridges Inventory Data," U.S. Dept. of Transportation, Federal Highway Admin., Office of Engineering, Bridge Division (1992), all of which are incorporated herein by reference.
Submarine Flowfields
The hull of a typical attack submarine is on the order of several hundred feet or more, and boundary layers of sizable extent develop along the surface of the hull. The various control surfaces that intersect the hull (e.g., submarine sail), cause a necklace vortex flow to form, and this localized vortical flow can be ingested into the propeller, particularly for the rearward control surfaces. This ingestion of the necklace vortex flow may produce acoustic noise and degrade stealth characteristics of submarine operations.
Architectural Aerodynamics
Pressure systems and localized wind gusts have long been a problem for architectural designs for outdoor cafes and meeting areas. When structures are subjected to oncoming air flows, unanticipated localized wind gusts may occur which are a problem for local pedestrian traffic and exterior design features.
When unobstructed wind travels over the ground, the speed of the wind is much lower at ground; in fact, the theoretical boundary conditions or no-slip condition states the speed is zero at the ground. The speed increases from the ground surface to above the ground, e.g., from zero at the ground to the freestream value of the wind far above the ground. This fluid mechanic event is known as the boundary layer which can be several tens or hundreds of feet high (e.g., the height of the layer from the ground to the point where the wind travels at the freestream value can be tens or hundreds of feet). When wind impacts the side of a building, a necklace vortex is formed, and the high energy flow in the upper part of the boundary layer is forced toward the ground. The necklace vortex can cause unsteady gusts of air near the ground which, among other things, are a problem for pedestrian comfort and functionality of exterior designs.
Large Scale Geological Flows
The occurrence of a necklace vortex system can be found at the foot of the windward side of mountains, hills, volcanoes or other geological or naturally-occurring obstacles. As discussed above, the necklace vortex is responsible for dragging high energy air flows from the upper part of the boundary layer to the ground level. In geological flows, these boundary layers can be several hundred feet high, and the necklace vortex can force atmospheric pollutants to the ground and create adverse health hazards for the public and wildlife, in addition to causing adverse wind conditions.
Aircraft Wing/Body Junctions, Inlet Struts, Rotor and Stator Blade Junctions
Aerodynamic shaped surfaces may be in contact with a surface having an engineering function. A necklace vortex system may be formed at wing/body junctions, engine inlets, and internal engine components which can be ingested into engines, thus degrading engine performance or, in some severe conditions, causing engine stall. The formation of necklace vortex systems are also responsible for drag penalties on aircraft as well as submarines.
Previous attempts at mitigating the effects of the necklace vortex have applied the intuitive approach of adding positive and negative vorticities to obtain zero. Structures such as strakes, fillets, dillets, and triangular-shaped ramp plates have been used to mitigate the necklace vortex Other efforts have included using staked washers, or large rocks placed around the pier (riprap), or vertical guide vanes protruding from the riverbed. See Richardson, Harrison, and Davis, "Evaluating Scour at Bridges," FHWA-IP-90-017, Hydraulic Engineering Circular No. 18, (February 1991) (incorporated by reference). None of these approaches have proven to be of significant long-term help in mitigating the scouring problem and adverse affects of the necklace vortex.
Structural retrofits have been suggested for other reasons. For example, U.S. Pat. No. 5,478,167 to Oppenheimer et al., "Buoyant Matter Diverting System," discloses a device placed in front of a structure to protect the structure by deflecting floating debris away from it. The device of Oppenheimer has a surface that is inclined to produce a negative lift and a wake downstream of the device and in front of the obstacle, with the downstream wake causing floating debris (e.g., ice, logs, trees, etc.), to deflect away from the structure (col. 3, 1. 40-50). The Oppenheimer patent does not address the necklace vortex or provide insight into floor physics or scour.
Thus, there is a need to reduce the effect of the necklace vortex on objects such as bridges, bridge-like piers and supports, submarines, architectural structures, aircraft, geological features, and other obstacles affected by the vortex.